Battery packs, each comprising a plurality of connected secondary batteries, have been widely used as the power source for mobile devices such as portable electronic appliances and electric vehicles. PTL 1 shows an example of a battery pack comprising a plurality of secondary batteries connected in series.
In order to provide a large capacity power source as the power source for a mobile device, a battery pack needs to include a unit comprising a large number of secondary batteries connected in parallel. Also, in order to provide a battery pack having a large output voltage, a large number of secondary batteries need to be connected in series.